drivers/serial/tcdrain: tcdrain() was recently added to the NuttX C library. ...

drivers/serial/tcdrain:  tcdrain() was recently added to the NuttX C library.  But there is a problem.  The specification of tcdrain() requires that it be a cancellation point.  In order to do this, tcdrain was moved from the C library into the OS and the addition cancellation point hooks were added.  In non-FLAT builds, access via system calls is also now supported.